Surface treatment for magnesiumlithium alloys



United States Patent 3,345,276 SURFACE TREATMENT FOR MAGNESIUM- LITHIUMALLOYS Robert A. Munroe, Candor, N.Y., assignor to InternationalBusiness Machines Corporation, New York, N.Y., a corporation of New YorkNo Drawing. Filed Dec. 23, 1963, Ser. No. 332,909 2 Claims. (Cl. 20432)The present invention relates broadly to articles of magnesium and itsalloys, and more particularly to a special treatment for such articlesoffering exceptional resistance to corrosion.

Magnesium and magnesium alloys have found wide acceptance as structuralmaterials where there is need for both relatively high strength andlight weight. A variety of alloying materials have been added tomagnesium in order to improve the mechanical strength properties ofsubstantially pure magnesium. One important class of such alloys are themagnesium-lithium alloys composed of 80% or more by weight of magnesium,and it is with particular reference to these that the invention derivesits major area of application.

Magnesium itself has a high resistance to atmospheric corrosion and moreresistance to alkali solutions than, say, aluminum. On the other hand,magnesiumis quite vulnerable to attack by most acids with the exceptionof chromic acids and hydrofluoric acids. With respect to salt sprayenvironments, the resistance against which is a common criterion ofchoice, magnesium is the most reactive of all the more commonly usedstructural metals. Alloying various materials, such as lithium, withmagnesium increases corrosion resistance from these sources; however,the alloys themselves are subject to deterioration accelerated by thepresence of cathodic impurities. Also, it is usually necessary to usemounting or joining means that are constructed of other metals(stainless steel, for example) forming galvanic couples that are quitedestructive of the magnesium alloys.

In view of the fact that galvanic couples are almost certain to beencountered in any useful design, special measures must be taken toprevent corrosion from this source. It is felt that organic coatingsoffer the best present answer to this problem and, as will be seen,their use represents an important aspect of the invention. Before such acoating can be applied to a magnesium alloy article, however, specialtreatment is necessary to insure a high degree of water resistance, goodadhesion of the coating to the metal surface, and flowing of theresinous material into minute crevices of the surface.

One known prior technique for this type of cleaning and surface treatingoperation is to immerse the articles in hydrofluoric acid. There are,however, practical difficulties with this, in that processing timeVaries considerably from the one workpiece to another and is notgenerally consistent for workpieces of the same geometry. Further, thereis no practical way in a production process of determining when cleaningis complete.

A more satisfactory method of surface cleaning or treatment for presentpurposes is what can be called fluoride anodizing which consistsgenerally of immersing the articles in an ammonium bifluoride solutionand with the articles serving as electrodes, apply an alternatingcurrent thereto. This cleans in the conventional sense by dissolvingand/or loosening surface impurities, and in addition forms a relativelyimpermeable protective surface layer or coating. An important beneficialadjunct from this process is that surface purification is obtainedwithout materially changing physical dimensions of the workpiece.Exemplary of this advantage in terms of another well-known prior arttechnique, to insure an equal degree of cleaning effect by moreconventional acid ice pickling methods (chromic and mineral acid baths,for example) may require the removal of as much as 0.040 inch of basemetal. It is an important aspect of the invention to provide an improvedprocedure for fluoride anodizing magnesium and magnesium alloy articles.

Exemplary of present known methods of cleaning magnesium alloy articlesby fluoride anodizing is that set forth in the United States Patent2,766,199, William F. Higgins the assignor to Magnesium ElektronLimited, Manchester, England.

It is therefore a primary object of the invention to provide a corrosionresistant surface to magnesium and magnesium alloys which is highlyadherent to the metal surfaces and possesses excellent qualities ofresistance particularly to electrolytic corrosion.

Another object is the provision of a tightly adherent layer on the metalsurfaces by an improved fluoride anodizing process.

Another object is the provision of a corrosion resistant surface as inthe above objects which is not destroyed by subsequent heating toelevated temperatures.

Still another object is the applying over the special corrosionresistant surface a resinous sealing coat that is tightly anduninterruptedly adherent thereto and substantially free from includeddeposits of dirt, moisture and released occluded gases.

Another object of the invention is the provision of a surface treatmentfor magnesium and its alloys which is highly simplified and readilyadapted to production manufacturing.

It is understood that the term magnesium metals" as used here refers notonly to substantially pure magnesium, but also to magnesium alloys ofall kinds. More particularly, the term relates to magnesium-lithiumwhich has exceptionally desirable physical characteristics, such asmechanical strength and toughness as well as the inherent light weightof magnesium.

Starting with a rough blank workpiece, for example, of a magnesium metalthe first step is to remove gross surface dirt, oil, grease and othersuch materials. This is best accomplished by the well-known process ofhot vapor degreasing. By this process the workpiece being treated isexposed to the hot vapor of a material such as perchlorothene forsufficient time to remove surface impurities. Time for removal isusually a matter of minutes at the most. 7

Mill scale can be removed next optionally by either immersion in anappropriate acid solution (10% nitric or sulphuric acids, for example)or by a suitable mechanical technique. The latter can be accomplished bythe application of dry pumice or by sanding, choice being dependent onthe particular construction of the. workpiece.

Both degreasing and scale removal, although important initial steps totake, are merely cleaning procedures preiiminary to anodizing and if themetal parts or articles are obtained originally in a sufficiently cleanform the following procedures may be begun directly.

Because of the relative instability of lithium, those alloys containinglithium require heating treatment prior to anodizing. That is, themagnesium-lithiumarticles are next placed in a circulating air, or inertgas, oven Where they are heated to a temperature that is m the range ofZOO-255 C. for at least one hour. This heating cycle serves to drive outor precipitate lithium from those portions of the alloy closely adjacentthe outer surfaces; As will be more fully appreciated later removing thelithium is an important preliminary step to obtaining the specialanodized surface of the invention.

After heating, the magnesium-lithium articles or parts are removed fromthe oven and permitted to cool to room temperature, with the rate ofcooling not being (3 critical for present purposes. The cooled articlesare immersed in continuously circulating water, which for best resultsshould be heated water not exceeding ap proximately .150 F., andmaintained therein for about 5 minutes. This washing operation removesthe precipitated lithium from the metal surfaces which if not done wouldinterfere with the subsequent fluoride anodizing.

After cleaning in the above-described manner, and the additional heatingstep in the case of magnesium-lithium, the parts are now in satisfactorycondition for forming the special fluoridized surface in accordance withthe present discovery. The parts are immersingly disposed in ananodizing bath consisting of an aqueous solution of ammonium bifiuoride(NH EHF), 30-45% by Weight. Temperature of the electrolyte is kept below27 C. during preparation and use. Specific gravity of the correctlyprepared solution will be within the range of 1.l651.170 at 24 C.

With suitable electrode bars clamped to the parts alternating current isapplied in a progressively increasing manner until 110-120 v. AC isreached. Voltage is maintained for 30-35 minutes after current falls toless than 5 a. AC per square foot of the parts surface. Throughout theanodizing a continuous relative movement between the parts andelectrolytic solution is provided to maintain the proper temperatureindicated above and to prevent localized overheating.

At completion of the anodizing, the parts are taken from theelectrolytic bath and thoroughly washed in running water to remove everytrace of the solution. Finally the, parts are dried by filteredcompressed air. Properly processed articles will possess a very uniform,clean, white or pearl gray finish. There will be no signs of attachedfoundry sand or other foreign matter, and completely free from darkareas indicating deficient anodizing caused, for example, by incompleteremoval of precipitated lithium in the case of magnesium-lithium alloys.

It is important to note the variation in type and character of thefluoridized surface formed by the above process on the different classesof alloys. With respect to those magnesium alloys that do not includelithium, the anodized coating formed is extremely thin (in order ofseveral millionths of an inch), and can be stripped from the base metalrelatively easily without damaging the base metal by the use of a 50% byweight chromic acid solution, for example. However, in the case ofmagnesium-lithium alloys a much thicker layer is formed which rangesfrom 0.00030.002 inch and this layer cannot be removed by chemicalstripping techinques without impairing the surface of the base meal. Itis possible to remove such coatings from the latter alloys by mechanicalmeans such as vapor honing.

'Further on this point, whereas the fluoridized coating on thenon-lithium magnesium alloys is a separate layer merely adhering in asurface-to-surface manner the coating on lithium alloys comprises aphysical change in the surface portions of the base metal itself. Thatis, in the latter case the anodized surface is substantially integral ina physical sense with the base metal. This can at least be partiallyexplained by the fact that during the special heating step preliminaryto anodizing, lithium is removed from the surface portions leaving smallcrevices or fissures which are filled during anodizing and thereby serveto anchor the coating and base metal together. It is felt, however, thatthis is not the only effect taking place, but rather there is anapparent penetration and intermingling of'the molecules of the anodiccoating with those of the magnesium metal itself forming a substantialregion of magnesium fluoride.

For full protection it is generally recognized that a resinous outercoating is required. Best results have been obtained where such outercoating is applied in successive layers until the desired thickness isobtained with each layer being dried and cured for that time andtemperature peculiar to the particular resin compound. Exemplary ofsatisfactory resinous coating compounds for this purpose are anepoxy-melamine manufactured under the tradename Hysol 4225, and apolyurethane termed Laminar X-500.

Many times it is desirable to provide a final coat of a suitable paint,or other finish, over the resinous coating. One such material excellentfor this purpose is Space Craft Coating XAl94.

The following specific examples serve to illustrate the invention, butthey are not intended to limit it thereto.

Example I A 4 x 4 x & inch sheet of magnesium alloy composed ofapproximately 84.5 percent magnesium, 14.0 percent lithium, 1.0 percentaluminum and 0.5 percent of trace metals and impurities was providedwith four (4) circular openings for receiving bolts therethrough.Initial cleaning comprised degreasing in hot perchlorothene vapor. Thesample was then introduced into a circulating air oven where it wasbrought to a temperature of 200 C. and held there for 1 hour. Thelithium exudation was removed by rinsing in 60 C. tap Water. Anodizingwas accomplished in a 30 percent ammonium bifluoride solution for 45minutes with an AC current of less than 1 ampere per square foot. Afterrinsing the anodized sheet was dried by maintaining at 55 C. for 1 hour,after which it was surface sealed with Laminar X-500 and cured at C. for1 hour. The seal also included the major surfaces and the edges with amaximum thickness of 0.5 mil. Stainless steel bolts were passed throughthe openings and nuts tightened thereon to insure good physical contactwith the resinous coating. The sample was subjected to a 20% salt sprayfor 50 hours and no signs of corrosion were in evidence.

Example 11 Identical to Example I except the final surface seal wasaccomplished with Hysol #4225. Testing included immersion in 3% saltsolution saturated with magnesium hydroxide (MgOH) for 50 hours. Theedges were left unsealed to determine the extent of any lateral movementof corrosion products under the seal. There was no corrosion notedexcept at the edges and this did not cause substantial peeling of theresin seal. A control panel of the same alloy, but completelyunprotected in any way, was subjected to the same test and completelydissolved.

Example 11] Panel and treatment were identical to Example 11 except thatthe edges were also sealed by the resin. In addition the panel waspainted with Space Craft Coating XA194. Immersion in 3% salt solutionsaturated with MgOH for 72 hours failed to produce any signs ofcorrosion.

In accordance with the practice of the invention described here anexceptionally tough and durable fluoride anodized coating can beprovided on magnesium and its alloys. It is believed that beneficialresults flow not only from the use of the special anodizing path, butalso particularly with respect to those alloys including lithium theprescribed heating and rinsing immediately prior to anodizing is asalient step in obtaining integration of the anodic coating with thebase metal of the character described.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein Without departing from the spirit and scope of theinvention.

What is claimed is:

1. In a fluoride anodizing process for surface treating articles ofmagnesium lithium alloys, the steps of:

exuding the lithium from those portions of the alloy closely adjacent tothe outer surfaces of said articles taminating and non-deterioratingrinse of continu ously circulating water having a temperature maintainedbelow 150 F., the removal of said lithium exudation being maintained forapproximately minutes while each of said articles is immersed in saidcirculating water; and

thereafter treating the articles by a fluoride anodizing process, saidfluoride anodizing process comprising: immersing the articles in anaqueous solution consisting essentially of 30-45% ammonium bifluoride;

applying an alternating current voltage to the articles and solution,and progressively increasing said voltage to 110-120 v. AC; and

maintaining the articles in the solution for at least 30 minutes afterthe current falls to less than 5 amperes per square foot of articlesurface.

maintained below 150 F., the removal of said lithium exudation beingmaintained for approximately 5 minutes while each of said articles isimmersed in said circulating water;

thereafter immersing the articles in an aqueous solution consistingessentially of 30-45% ammonium bifluoride;

applying an alternating current voltage to the articles and solution,and progressively increasing said voltage to 110-120 v. AC; and

maintaining the articles in the solution for at least 30 minutes afterthe current falls to less than 5 amperes References Cited UNITED STATESPATENTS per square foot of article surfac 552%? 323? 5:2: 5 2 55? 2. Amethod of surface treating metal a icl s of m 2428749 10/1947 De u 148 62 nesium lithium alloys comprising the Steps 0ft 2459744 1/1949 R ft r gu 318 28 pretreating the articles by a non-contaminating and 2692213 54117 222 non-deteriorating cle ning pr 2766199 10/1956 Hi g 204 X exudingthe lithium from those portion of the l y 2885315 5/1959 -i n 148 20 3closely adjacent to the outer surface of said articles 2961359 11/1960-h- 1 by raising and maintaining the temperature of said 16 e a u Iarticles to a temperature of the range ZOO-255 C 3012917 12/1961 Rlou eta1 117-145 3,032,435 5/1962 Michel 117-7() for at least one hour in anon-contaminating and hnon-deteriorating atmoslpllllere; h JOHN H. MACK,Primary Examiner. t ereafter removing the it ium so exuded from t esurfaces by cooling said articles to room temperature HOWARD WILLIAMSExammer' and then washing the cooled articles in a non-con- G. KAPLAN,Assistant Examiner.

2. A METHOD OF SURFACE TREATING METAL ARTICLES OF MAGNESIUM LITHIUMALLOYS COMPRISING THE STEPS OF: PRETREATING THE ARTICLES BY ANON-CONTAMINATING AND NON-DETERIORATING CLEANING PROCESS: EXUDING THELITHIUM FROM THOSE PORTIONS OF THE ALLOY CLOSELY ADJACENT TO THE OUTERSURFACE OF SAID ARTICLES BY RAISING AND MAINTAINING THE TEMPERATURE OFSAID ARTICLES TO A TEMPERATURE OF THE RANGE 200-255*C. FOR AT LEAST ONEHOUR IN A NON-CONTAMINATING AND NON-DETERIORATING ATMOSPHERE: THEREAFTERREMOVING THE LITHIUM SO EXUDED FROM THE SURFACES BY COOLING SAIDARTICLES TO ROOM TEMPERATURE AND THEN WASHING THE COOLED ARTICLES IN ANON-CONTAMINATING AND NON-DETERIORATING RINSE OF CONTINUOUSLYCIRCULATING WATER HAVING A TEMPERATURE MAINTAINED BELOW 150* F., THEREMOVAL OF SAID LITHIUM EXUDATION BEING MAINTAINED FOR APPROXIMATELY 5MINUTES WHILE EACH OF SAID ARTICLES IS IMMERSED IN SAID CIRCULATINGWATER; AND THEREAFTER TREATING THE ARTICLES BY A FLOURIDE ANODIZINGPROCESS, SAID FLUORIDE ANODIZING PROCESS COMPRISING: IMMERSING THEARTICLES IN AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF 30-45%AMMONIUM BIFLUORIDE; APPLYING AN ALTERNATING CURRENT VOLTAGE TO THEARTICLES AND SOLUTION, AND PROGRESSIVELY INCREASING SAID VOLTAGE TO110-120 V. AC; AND MAINTAINING THE ARTICLES IN THE SOLUTION FOR AT LEAST30 MINUTES AFTER THE CURRENT FALLS TO THE LESS THAN 5 AMPERES PER SQUAREFOOT OF ARTICLE SURFACE.